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Detecting Nature’s Gentlest Particles

Q&A with Dr. Gilles Gerbier

Dr. Gilles Gerbier is a professor in the Department of Physics, Engineering Physics and Astronomy at Queen’s University. Since helping found the Beijing-Paris-Rome-Saclay Collaboration in Europe, Gerbier has been working on the design of a dark matter detector, and has brought his expertise in particle astrophysics and collaborative research to Queen’s as the Canada Excellence Research Chair in Particle Astrophysics. Lowell Cochrane visited Gerbier at Queen’s to ask him about his work and dark matter experiments.

Q: How did you become interested in particle astrophysics and dark matter physics in particular?

A: Earlier in my career, I had a chance to pursue research into high energy particle physics. I obtained my PhD from the Université Paris, conducting research at Saclay (Institut de Physique Théorique) where I did work on high energy physics at the beam at CERN, studying quark – neutrino interactions in bubble chambers. After that, I went on to study proton decay experiments at my first underground experience at the Modane Underground Laboratory, which is situated in a mountain tunnel between France and Italy. Afterwards, I held a postgraduate fellowship at Berkeley where I did work specifically on particle astrophysics. I then returned to Saclay, where I was part of an international collaboration using scintillator detectors to study dark matter.

Q: Given that there is so much media about high energy physics, particularly with the Large Hadron Collider at CERN, what was it about low energy physics that attracted you?

A: Where high energy physics looks at fundamental matter using larger and larger particle accelerators, I felt that dark matter research offered a more creative path for the design of experiments, where I could set my mind towards coming up with novel approaches for detecting WIMPs.

Q: Can you explain what dark matter is, and what a WIMP is in particular?

A: Dark matter is one of the greatest mysteries in physics. Astrophysical observations of the way stars rotate around galaxies, and the way galaxies interact with one another, and even the way light is bent as it moves suggests that there is more gravity at play than one would expect from just the visible matter. We know that this matter – called “dark” because it doesn’t shine like regular matter and because we know so little about it – is not like ordinary matter. It is what physicists call “weakly interacting,” meaning “barely detectable.”

So we think that dark matter must be made up of a certain kind of particle, which we call a Weakly Interacting Massive Particle (WIMP). We say they are massive to distinguish them from other weakly interactive particles, like neutrinos, which have very little mass and don’t explain the missing mass of dark matter.

Q: If WIMPs are so difficult to detect, and you aren’t sure that they exist, how do you go about detecting them?

A: Well, it is true that so far no detector has detected a dark matter particle, at least not with any confidence. However, from the experimenter’s point of view, if they exist, they should sometimes reveal their presence under very precise conditions and very careful observation. Any detector that we build will make certain assumptions about WIMPs, including how massive we think they are. To date, nearly all experiments have looked for WIMPs in mass ranges of multiples of the mass of a proton. We have now eliminated those ranges. The experiments I’m working on will look at much lower ranges.

Q: Is it more difficult to detect WIMPs if they have a lower mass?

A: Yes, in a way. The proposed detectors will have to be extremely sensitive, and they will need to be shielded from natural cosmic radiation and radioactivity that would create far too much noise to be able to see a WIMP. But we have designed some very sensitive detectors, and we have the perfect low-noise conditions in the very deep and clean SNOLAB, 2km underground in Vale’s Creighton Mine in Sudbury.

Q: Can you describe the detectors that you’re working on at the moment?

A: The first is a cryogenic detector, named SuperCDMS (Cryogenic Dark Matter Search). It is made up of stacks of crystalline Germanium that we keep at temperatures very close to absolute zero. By keeping the substance at such a low temperature, any dark matter particles interacting with the Germanium will cause a very small, but detectable increase in temperature in the detector.

The second experiment called NEWS (New Experiments With Spheres) is a spherical gas detector. A large copper sphere will contain helium gas and a number of sensitive and precisely positioned electrodes, which can detect interactions between any WIMPs and the nuclei of the gas atoms.

Q: Does collaboration play an important role in all your work?

A: In all of these experiments, the components that come together to make the experiments work come from different teams spread out over the globe – from the US, to Greece, to France, and of course, Canada. One of the reasons I came to Canada in this position was to help build connections between research teams across the globe.

Q: Just how big a discovery would it be to be able to detect dark matter?

A: Whenever we are able to look at a part of reality that has never been seen before, there is always the potential to revolutionize particle physics. The detection of a new kind of matter, particularly a form of matter predicted to make up 27% of the universe, would certainly be a great discovery for everyone.